Cellular telephones may operate under a variety of standards including the code division multiple access (CDMA) cellular telephone communication system as described in TIA/EIA, IS-95, Mobile station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, published July 1993. CDMA is a technique for spread-spectrum multiple-access digital communications that creates channels through the use of unique code sequences. In CDMA systems, signals can be and are received in the presence of high levels of interference. The practical limit of signal reception depends on the channel conditions, but CDMA reception in the system described in the aforementioned IS-95 Standard can take place in the presence of interference that is 18 dB larger than the signal for a static channel. Typically, the system operates with a lower level of interference and dynamic channel conditions.
A mobile station using the CDMA standard constantly searches a Pilot Channel of neighboring base stations for a pilot that is sufficiently stronger than a pilot add threshold value T_ADD. As the mobile station moves from the region covered by one base station to another, the mobile station promotes certain pilots from the Neighbor Set to the Candidate Set, and notifies the base station or base stations of the promotion from the Neighbor Set to the Candidate Set via a Pilot Strength Measurement Message. The base station determines an Active Set according to the Pilot Strength Measurement Message, and notifies the mobile station of the new Active Set via a Handoff Direction Message. The mobile station will maintain communication with both the old base station and the new base station so long as the pilots for each base station are stronger than a pilot drop threshold value T_DROP. When one of the pilots weakens to less than a pilot drop threshold value, the mobile station notifies the base station of the change. The base station may then determine a new Active Set, and notifies the mobile station of that new Active Set. Upon notification by the base station, the mobile station then demotes the weakened pilot to the Neighbor Set.
Switching pilots into and out of the Active Set of a mobile station causes several problems with the handoff. First, the mobile station may have several pilots active at the same time. Second, the system capacity is decreased. Third, excessive signaling between the mobile station and base stations results from the increased number of handoff messages. It is desired to provide a method and system that reduces the switching of pilots into and out of the Active Set of a mobile station.